1. Field of the Invention
This invention relates to a volatile gas detector and particularly to a portable photo-ionization detector (PID).
2. Description of Related Art
FIG. 1 illustrates a known photo-ionization detector (PID) 10 for detecting volatile gases. PID 10 includes an ultraviolet (UV) lamp 12, an ion detector 18 and a UV monitor 26. In operation , UV lamp 12 produces high-energy photons having energy above 9.2 electron volt (eV) which emanate through an optical window 16 into an ionization chamber 14. In ionization chamber 14, the UV photons collide with gas molecules including volatile gas having ionization potentials below the energy of the UV photons. This ionizes the volatile gas molecules, creating detectable ions and electrons.
Ion detector 18 includes a negative electrode 20 and a positive electrode 22 which have a high voltage difference (e.g., greater than 150 V). Accordingly, negative electrode 20 attracts positively charged particles such as ions, and positive electrode 22 attracts negatively charged particles such as electrons. As a result, the production of volatile gas ions causes a current from electrode 22 to electrode 20 that depends on the number of ions produced. The concentration of the volatile gases in ionization chamber 14 can be determined by measuring the current and the intensity of UV light. At a constant UV light intensity, the measured current is nearly proportional to the volatile gas concentration, and the measured current can be simply converted to the concentration, in parts per million (ppm), of the volatile gases.
PID 10 has a space 24 between optical window 16 and positive electrode 22. Space 24 is a "dead zone" in which positive ions can be trapped. The positive polarity of electrode 22 prevents positive ions in space 24 from reaching electrode 20. Accordingly, the configuration of electrodes 20 and 22 with dead space 24 inhibits the collection of ions and can reduce the range and sensitivity of PID 10. For example, PID 10 typically has a detection range of about 2,000 ppm of ionizable gases.
As mentioned above, the measured current can be simply converted to a concentration of volatile gases if the UV intensity from lamp 12 remains constant. However, the UV intensity typically diminishes during a normal operation of PID 10 due to a variety of factors, including degradation of UV lamp 12, contamination of optical window 16 and the presence of interfering substances such as methane, carbon monoxide or water which block or absorb the UV photons in ionization chamber 14. UV monitor 26, which includes a negatively biased electrode, measures the intensity of the UV light by measuring a current caused by the photoelectric effect of the UV light. In particular, when struck by the UV photons, UV monitor 26 releases electrons which cause a monitor current indicative of the intensity of the UV light. The monitor current can be measured to determine UV intensity variations when calculating the volatile gas concentration. The monitor current can also be used when adjusting the intensity of UV lamp 12, for example, by increasing a supply voltage to lamp 12 in response to the monitor current indicating a low UV intensity. However, the presence of ionizable gases around UV monitor 26 increases the monitor current because a positive electrode of UV monitor 26 also collects positive ions. Accordingly, the monitor current inaccurately measures the UV intensity. Absorption of the UV light along the path from UV lamp 12 to UV monitor 26 further reduces the accuracy of the monitor current as an indicator of the UV intensity. Therefore, a PID that can accurately measure the UV intensity, is needed.